This power is a very mysterious one, and of a nature to perplex even the philosophic observer. Certain bodies, such as the metals, convey it, and are called conductors; certain others, such as glass and porcelain, arrest it, and are called insulators. It is for this reason that the wires of the telegraph are supported by a non-conductor, for if not, the electric current would pass into the earth by the first post and never reach its final destination. Glass being an insulator, it was found that, if a glass bottle was filled with water, and then corked up with a cork, through which a nail was passed so that the top of it touched the water, it would receive and retain a charge as long as it was held in the hand; and this observation led to an invention of some account in the subsequent applications of electricity, known, from the place of its conception, as the Leyden jar. This is a glass jar, the inside of which is coated with tinfoil, and the outside as far as the neck, and into which, so as to touch the inside coating, a brass rod with a knob at the top is inserted through a cork, which closes its mouth. By means of this, in consequence of the isolation of the coatings by the glass, electricity can, in a dry atmosphere, be condensed, and stored up and husbanded till wanted. A series of eggs, arranged in contact and in line, give occasion to a pretty experiment. In consequence of the shells being non-conductors, and the inside conducting, it happens that a current of electricity, applied to the first of the series, will pass from one to another in a succession of crackling sparks, in this way forcing itself through the obstructing walls. This effect of electricity in making its way through non-conducting obstructions accounts for the explosion which ensues when a current of it comes in contact with a quantity of gunpowder; as it also does for the fatal consequences which result when, on its way from the atmosphere to the earth, it rushes athwart any resisting organic or inorganic body. _Magnetism_.--Unlike electricity, which acts with a shock and then expires, magnetism is a constant quantity, and constant in its action; and it has this singular property, that it can impart itself as a permanent force to bodies previously without it. Thus, there being natural magnets and artificial, we can, by passing a piece of steel over a magnet, turn it into a strong magnet itself; although we can also, when it is in the form of a horse-shoe, by a half turn round and then rubbing it on the magnet, take away what it has acquired, and bring it back to its original state. The magnetic property is very readily imparted (by induction, as it is called) to soft iron, but when the iron is removed from the magnetising body, it parts with the virtue as fast as it acquired it. To obtain a substance that will retain the power induced, we must make some other election; and hard steel is most serviceable for conversion into a permanent magnet.